This invention relates to an improved protective coating composition. More particularly, it relates to a silicone resin coating composition which, when applied to a substrate, forms a protective abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is not utilized in public transportation vehicles, such as brains, buses, taxis and airplanes. Lenses, such as for eyeglasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires placement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated.
Attempts have been made to improve the abrasion resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997, 3,976,497 and 4,177,315, for example, describe such compositions.
Copending U.S. application entitled "Silicone Resin Coating Compositions," by Howard A. Vaughn, Ser. No. 964,910, filed Nov. 30, 1978, discloses another abrasion-resistant coating composition. Copending Frye application, Ser. No. 964,911, filed Nov. 30, 1978, describes the use of small amounts of polysiloxane polyether copolymers to promote coating formation. Copending Frye application, Ser. No. 91,716, filed Nov. 6, 1978, describes the use of small amounts of ultraviolet screens to improve adhesion on weathering. Such coating compositions have been found to be highly desirable as a protective finish for plastic, as well as metal or metallized substrates. A particularly significant area of application for those improved coatings is in the glazing and optical lens industry. All such coatings generally require a thermal cure for about one hour at 120.degree. C. to achieve acceptable abrasion resistance. It would be desirable to lower this cure temperature, while retaining adequate adhesion. It has now been surprisingly discovered herein that the addition of small amount of a .beta.-hydroxyketone compound to the coating compositions disclosed in the aforementioned patents and especially those of the applications eliminates the need to cure at such high temperatures, while retaining adhesion properties as well as providing other improvements in the hard coating which will be discussed in detail hereinafter. The patents and applications are incorporated herein by reference.